Known among onboard navigation aid systems for aircraft following trajectories relatively close to the ground, are ground collision risk warning systems termed TAWS (acronym standing for the expression: “Terrain Awareness and Warning System”). These TAWS systems emit alarms regarding the need to modify, in the shorter or not so short term, the trajectory followed so as to avoid a collision with the relief and obstacles on the ground, or the penetration into a forbidden overfly zone. They base their alarms on the penetration of the relief or of an obstacle on the ground or the passage of a forbidden overfly zone, into or under one or more protection volumes related to the aircraft and extending ahead of and below the current position of the aircraft. The relief, the obstacles on the ground and the forbidden overfly zones are catalogued in a topographic map of the region overflown formulated on the basis of a terrain elevation database onboard or consultable from the aircraft. The protection volume or volumes related to the current position of the aircraft are dimensioned so as to contain the majority of the disengagement trajectories within the range of the aircraft, in relation to a possible relief, obstacle or forbidden overfly zone situated on its short-term forecastable trajectory.
These TAWS ground collision risk warning systems are only concerned with ground collision risks presented by the short-term forecastable trajectory of the aircraft, deduced from its current position and its speed and acceleration vectors at the time. They do not advise the crew as to its lateral margins of maneuver.
To respond to this requirement, the Applicant has already proposed, in a French patent application FR 2.842.594, an onboard navigation aid system for aircraft alerting the crew of an aircraft as to the time remaining for it to embark on lateral disengagement maneuvers, when there appear restrictions to its lateral margins of maneuver on either side of a fictitious trajectory such as its short-term forecastable trajectory. More precisely, this system implements a method consisting in:                defining two zones to be probed, one to the right and the other to the left, of a fictitious trajectory to be traversed, designated by prober support trajectory,        probing, for each of the two zones to be probed to the right and to the left, a subjacent relief with a view to identifying dangerous sub-zones to the right and/or to the left,        calculating for each of the dangerous sub-zones to the right and/or to the left, a time ΔT remaining to start a disengagement maneuver before a point of no return, and determining for the dangerous sub-zones to the right a minimal ΔT denoted ΔT right and/or for the dangerous sub-zones to the left a minimal ΔT denoted ΔT left, and        establishing a navigation aid on the basis of ΔT right and/or ΔT left.        